Coating compositions for casting moulds and cores for avoiding maculate surfaces

ABSTRACT

The invention relates to a size composition (coating composition) for casting moulds and cores, comprising at least one metal additive which contains a metal or a compound of a metal, wherein the metal is selected from one of groups 7 or 9 to 12 of the Periodic Table of the Elements. The invention also relates to a process for producing a casting mould, which comprises a mould coating of the size (coating) according to the invention, and to the use of said mould for the casting of metals.

The invention relates to a size (coating), which is particularly adapted for mass casting, a method for producing a cast part and a casting mould comprising a mould coating.

Most products of the iron and steel industry, as well as those of the nonferrous metal industry undergo casting for the first shaping. The molten materials, either ferrous metals or nonferrous metals, are converted into geometrically defined objects with defined workpiece properties. In order to shape the cast parts, very complicated casting moulds must sometimes first be produced to receive the melts. The casting moulds are divided into dead moulds, which are destroyed after every casting process, and permanent moulds, with each of which a large number of cast parts can be produced.

The dead moulds usually consist of a mineral, refractory, granular mould material that is often mixed with various further additives, for example in order to achieve a good casting surface that is solidified with the aid of a binder. Washed, graded silica sand is usually used as a refractory, granular mould material. Chromite, zircon and olivine sand are also employed for specific applications in which particular requirements must be satisfied. In addition, mould materials based on fireclay, as well as magnesite, silimanite or corundum are still used. The binder with which the mould materials are solidified can be of an inorganic or organic nature. Smaller dead moulds are predominantly produced from mould materials that are solidified by bentonite as a binder, whilst organic polymers are usually used as a binder for larger moulds. The production of casting moulds is usually carried out by first mixing the mould material with the binder in such a way that the particles of the mould material are coated with a thin film of the binder. This mould material mixture is then introduced into a suitable mould and optionally compressed to achieve sufficient stability of the casting mould. The casting mould is then cured, for example by being heated or by adding a catalyst. Once the casting mould has reached at least a specific initial strength, it can optionally be removed from the mould and for complete curing be transferred to a kiln for example in order to be heated there for a specific period of time to a specific temperature.

Permanent moulds are used for the production of a large number of cast parts. They must therefore withstand the casting process and the associated loads without becoming damaged. Depending on the field of application, particular cast irons as well as unalloyed and alloyed steels, and also copper, aluminium, graphite, sintered metals and ceramic materials have proven to be effective materials for permanent moulds. Gravity die casting, high-pressure die casting, centrifugal casting and continuous casting methods are examples of permanent mould casting.

Casting moulds are subjected to very high thermal and mechanical loads during the casting process. Faults may therefore be produced at the contact face between liquid metal and the casting mould, for example where the casting mould cracks or liquid metal penetrates into the structure of the casting mould. The faces of the casting mould that come into contact with the liquid metal are usually provided with a protective coating that is also referred to as a size. A size of this type usually consists of an inorganic refractory material and a binder that are dissolved or suspended in a suitable solvent, for example water or alcohol.

The surface of the casting mould can thus be modified by these coatings and adapted to the properties of the metal to be processed. The appearance of the cast part can thus be improved by the size by producing a smooth surface since irregularities caused by the size of the particles of the mould material are compensated for by the size. The size can furthermore influence the cast part metallurgically, for example by selectively transferring additives at the surface of the cast part into the cast part via the size, these additives improving the surface properties of the cast part. Furthermore, the sizes form a layer that chemically insulates the casting mould from liquid metal during casting. Any adhesion between the cast part and the casting mould is thus prevented in such a way that the cast part can be removed from the casting mould without difficulty. In addition, the size ensures a thermal separation between the casting mould and the cast part. This is particularly important in the case of permanent moulds. If this function is not fulfilled, a metal mould for example will be subjected to such high thermal loads during the successive casting processes that it will be destroyed prematurely. However, the size can also be used to selectively control the heat transfer between the liquid metal and casting mould, for example in order to form a specific metal structure as a result of the cooling rate.

The sizes used conventionally contain, for example, clays, quartz, diatomaceous earth, cristobalite, tridymite, aluminium silicate, zirconium silicate, mica, fireclay or else coke or graphite as raw materials. These raw materials cover the surface of the casting mould and seal the pores against penetration of the liquid metal into the casting mould. As a result of their high insulating capability, sizes that contain silicon dioxide or diatomaceous earth as raw materials are often used since these sizes can be produced at low cost and are available in large volumes.

It has already been attempted to selectively introduce alloy constituents into the surface of a cast part via a size layer, for example in order to improve the hardness of the surface. K. Herfurth and S. Pinkert, Technische Zeitschrift fur das Gieβereiwesen, 19, 1973, 365-400, as well as K. Herfurth, S. Pinkert, K. Nowak, “Oberflächenlegierungen von Stahlguss in der Gieβform”, Freiberger Forschungsbericht, B 184 1975, 203-215 thus describe pastes that contain large amounts of transition metals such as chromium, nickel or manganese in the form, for example, of ferrochromium or ferromanganese. The fraction of these metals or alloys in the paste is considerably more than 50 wt. %. In addition, a binder is also contained in the paste, normally water glass. These pastes are applied to faces of the casting mould that come into contact with the liquid metal material, normally steel, during casting. During casting the metals contained in the paste are melted by the heat of the liquid metal material and form an alloy therewith locally at the surface of the cast part, this alloy then setting as a peripheral shell. Depending on the way in which the process is carried out, peripheral shells with a thickness of up to 10 mm may be produced. These peripheral shells may thus be very hard. In order to produce a casting mould, for example a dredging shovel, it would no longer be necessary with this method to produce the entire cast part from the corresponding alloy. Instead it would be sufficient to selectively produce merely the portions of the casting mould that are particularly loaded, for example the teeth of the dredging shovel, with the aid of the alloy constituents contained in the paste during the casting process from an alloy that makes it possible to achieve particularly high surface hardness. However, what is problematic in this method is the different shrinkage coefficient of the various materials. The cured surface layer does not therefore have the same thickness throughout or else irregularities in the surface are observed, such as cracks, chips or recesses.

In iron and steel casting, faults sometimes form at the surface of the cast part, such as a maculate, uneven or burnt-in surface, chips, pitting, holes or pinholes or white or black coatings are formed. The causes of these faults are not yet fully understood. It has been attempted to combat them, for example by changing the casting parameters, modifying the binder system of the casting mould or else adding various additives to the sizes. However the success of these measures has generally been unsatisfactory. The fault does initially disappear. However it reappears after some time, although it is impossible to predict the timing, intensity and size of the fault.

If the aforementioned faults do occur, complex reworking of the surface of the cast part is necessary in order to achieve the desired surface properties. This requires addition process steps and therefore a reduction in productivity or an increase in costs. If the faults occur at faces of the cast part that are not easily accessible or are inaccessible, this can also lead to wastage of the cast part.

The object of the invention is therefore to propose measures that can be used in metal casting to improve the surface of the cast part in such a way that the extent of surface treatment of the cast part after casting can be reduced.

This object is achieved with a size composition having the features of claim 1. Advantageous developments of the size composition according to the invention are the subject of the dependent claims.

It has surprisingly been found that adding a specific metal additive to a size composition can lastingly improve the quality of the surface of the cast part, and for example the formation of graining at the surface of the cast part is largely or completely suppressed. It is not observed that larger amounts of the metal additive or its constituents pass into the cast part. There are therefore no difficulties caused by different expansion or shrinkage coefficients between the cast part and peripheral shell.

In accordance with the invention the metal additive contained in the size composition contains at least one metal or a compound of a metal, the metal being selected from one of groups 7 or 9 to 12 of the Periodic Table of the Elements.

The groups are numbered based on the currently applicable rules of the IUPAC. In accordance with the old rules of the IUPAC, group 7 corresponds to group VIIA. Groups 9 and 10 correspond to the elements Co, Rh, Ir as well as Ni, Pd and Pt of the old group VIIIA and groups 11 and 12 correspond to groups IB or IIB of the old notation.

The metal additive can contain a metal, i.e. the metal in the oxidation state zero, the metal possibly being employed both in pure form and in the form of an alloy with other metals. However, the metal additive can also be present in the form of an oxidised metal, i.e. in the form of an oxide or a salt, such as a carbonate, a nitrate or chloride, the oxide being preferred.

The metal is preferably employed in reduced form, i.e. in the oxidation state zero.

A plurality of the named metals or compounds of these metals can be contained in the metal additive. However, only one of the metals is preferably contained in the metal additive in reduced or oxidised from.

Metals or their compounds selected from groups 7, 10 or 11 of the Periodic Table of the Elements are preferably used in the metal additive, manganese, nickel and copper being particularly preferred.

The metal additive may be formed merely of the named metals or their compounds. However it is also possible that further metals or compounds are contained in the metal additive in addition to these metals or their compounds.

In accordance with a preferred embodiment the metal or metal compound, calculated as metal and based on the weight of the metal additive, is contained in the metal additive in an amount of at least 10 wt. %, especially in an amount of at least 20 wt. %, preferably in an amount of at least 30 wt. %, particularly preferably in an amount of at least 40 wt. % and more particularly preferably in an amount of at least 50 wt. %.

In accordance with one embodiment the metal additive is only formed of at least one of the named metals, particularly manganese, nickel or copper. However, in accordance with one embodiment it is sufficient for the metal or its compound to be contained in the metal additive in an amount of less than 90 wt. %, in accordance with a further embodiment in an amount of less than 80 wt. % and in accordance with yet a further embodiment in an amount of less than 70 wt. %.

In addition to the metal additive, the size composition can also contain further constituents that are conventional for sizes. In accordance with a preferred embodiment the metal additive is contained in the size composition in an amount, based on the solid fraction of the size composition, of at least 10 wt. %, preferably at least 15 wt. % and particularly preferably at least 20 wt. % in order to achieve a lasting influence on the surface of the cast part.

As already explained, the size is not preferably used to achieve an alloy of a surface layer of a cast part, but instead so the surface or a peripheral shell has substantially the same composition as portions of the cast part that are arranged spaced from the surface of the cast part, i.e. in its volume.

It is therefore preferably provided for the fraction of metal additive in the size composition to be selected as less than 50 wt. %, especially less than 40 wt. % and particularly preferably less than 35 wt. % based on the solid content of the size composition.

As already explained, the metal additive may contain merely at least one of the above-mentioned metals, preferably at least one metal of manganese, nickel and copper. However, in accordance with one embodiment it is also possible for the at least one metal to be contained in the metal additive in the form of an alloy. In accordance with one embodiment the metal is contained in the metal additive in the form of an iron alloy. The fraction of iron in the metal additive, expressed as elemental iron, is preferably selected in the range from 20 to 80 wt. %, especially 30 to 70 wt. %.

In addition to the metal and the iron, the alloy may also contain further constituents.

In accordance with a further embodiment the metal additive contains aluminium as a constituent, the fraction of aluminium in the metal additive, determined as elemental aluminium, is selected as especially less than 10 wt. % and preferably less than 8 wt. %. In accordance with one embodiment the metal additive contains aluminium in a fraction of more than 2 wt. %. In accordance with one embodiment of the size according to the invention the metal additive comprises a fraction of aluminium in the range from 2 to 8 wt. %, preferably 3 to 6 wt. %, particularly preferably 3 to 5 wt. %.

In accordance with one embodiment the metal additive can also be used in a silicon alloy. The silicon fraction of a silicon alloy of this type is preferably selected in a range from 20 to 80 wt. %, particularly preferably 50 to 70 wt. %.

The metal additive may comprise yet further constituents, in particular metals, the fraction thereof especially being selected as less than 2 wt. %, preferably less than 1 wt. %.

These further constituents are preferably selected from the group of cerium, magnesium, chromium and molybdenum.

The fractions of these alloy constituents are preferably between 0.01 and 2 wt. %, preferably 0.1 to 1 wt. % based on the metal additive. The metal additive may also contain calcium as a further alloy constituent. The content of calcium is preferably in the range from 0.2 to 2 wt. %, particularly preferably 0.5 to 1.5 wt. %.

The particle size of the metal additive should preferably not be selected to be too small, particularly if the metals, preferably manganese, nickel and copper, are contained in the metal additive in elemental form since there is then an increased risk that the metal additive will react with further constituents of the size composition and, for example, become oxidised. On the other hand, the particle size should preferably not be selected to be too large, since otherwise the metal additive may, for example, sink in the size composition and therefore the metal additive will be applied unhomogeneously over the face of a casting mould.

The metal additive preferably has a mean particle size (D₅₀) of less than 0.5 mm, preferably less than 0.4 mm, particularly preferably less than 0.3 mm. The mean particle size (D₅₀) can be ascertained, for example, by screen analysis or by laser granulometry. The metal additive contained in the size according to the invention usually has a relatively high density and thus sinks quickly in the size. However, this sinking can be decelerated by adding a floating agent. The sinking of the inoculum can also be reduced further by decreasing the particle size, in such a way that the inoculum remains suspended homogeneously in the size. As a further advantage, when using a spraying device to apply the size, the nozzle of the spraying device is less easily blocked when using a metal additive with a fine particle size. The inoculum particularly preferably has a mean particle size of less than 0.3 mm. With decreasing particle size however, the specific surface of the metal additive increases and therefore reactivity with the liquid contained in the size, for example water, also increases. In the case of a reaction of the metal additive with water for example, gas formation is observed that leads to foam formation. The size can no longer be reliably pumped or sprayed. The mean particle size is therefore preferably selected to be greater than 50 μm, particularly preferably greater than 80 μm. The metal additive is preferably used with a particle size in the range from 20 to 1000 μm, more preferably from 80 to 300 μm.

The size composition according to the invention is preferably provided in the form of a paste or a suspension. In this embodiment the size composition contains a carrier liquid. This carrier liquid is suitably selected in such a way that it can be completely evaporated in the conditions that prevail conventionally during metal casting. The carrier liquid should therefore preferably have a boiling point at normal pressure of less than approximately 130° C., preferably less than 110° C.

The carrier liquid may be formed in part or completely of water. However, oxidation of the metal additive may be observed, particularly if the metal additive is present in the form of elemental metals or an alloy of elemental metals. In accordance with a preferred embodiment the size composition thus contains a solvent that is formed at least in part of an organic solvent.

The oxidation of the metal additive is repressed by a high fraction of organic solvent, for example an alcohol. As a further advantage the size can be dried very easily after application by burning off the solvent.

If an organic solvent is contained in the size composition, the fraction thereof in the carrier liquid is especially selected to be greater than 20 wt. %, preferably greater than 30 wt. % and particularly preferably greater than 40 wt. %.

The carrier liquid may be formed completely of the organic solvent. However, in accordance with one embodiment the fraction of organic solvent in the carrier liquid may also be selected to be lower. In accordance with one embodiment the fraction of organic solvent in the carrier liquid is less than 90 wt. %, in accordance with a further embodiment less than 80 wt. % and in accordance with a further embodiment less than 70 wt. %.

Examples of suitable solvents include aliphatic, cycloaliphatic or aromatic hydrocarbons that preferably comprise 5 to 15 carbons, or esters of aliphatic carboxylic acids in which the carboxylic acids preferably comprise 2 to 20 carbon atoms and the alcohol constituent of the ester preferably comprises 1 to 4 carbon atoms. Examples of further preferred organic solvents include ketones with preferably 4 to 20 carbon atoms. Ethers are also suitable as a solvent, in this instance it also being possible to use polyglycols.

In accordance with a preferred embodiment the solvent is at least partly formed of at least one alcohol that preferably comprises 1 to 10 carbon atoms. Exemplary alcohols are ethanol, n-propanol, isopropanol and butanol.

If an alcohol is used as a constituent of the carrier liquid, the fraction thereof based on the weight of the carrier liquid is preferably selected to be greater than 50 wt. % and preferably greater than 60 wt. %.

In addition to the constituents already named, the size composition according to the invention may also contain further constituents that are conventional for sizes.

In accordance with a preferred embodiment the size composition according to the invention thus comprises at least one pulverulent refractory material. This refractory material seals the pores in a casting mould against the penetration of the liquid metal. Thermal insulation between the casting mould and liquid metal is further achieved by the refractory material. Refractory materials that are conventional in metal casting can be used as a refractory material. Examples of suitable refractory materials include quartz, aluminium oxide, zirconium oxide, aluminium silicates such as pyrophyllite, kyanite, andalusite or fireclay, zircon sand, zircon silicates, olivine, talc, mica, graphite, coke, feldspar, diatomaceous earth, kaolin, calcined kaolin, kaolinite, metakaolinite, iron oxide and bauxite.

The refractory material is provided in powder form. The particle size is selected in such a way that a stable structure is produced in the coating and the size can preferably be distributed over the wall of the casting mould with a spray device in a trouble-free manner. The refractory material suitably has a mean particle size in the range from 0.1 to 500 μm, particularly preferably in the range from 1 to 200 μm. In particular, materials that have a melting point at least 200° C. above the temperature of the liquid metal and that do not react with the metal are suitable as a refractory material.

The fraction of refractory material based on the solid fraction of the size composition is especially selected to be greater than 10 wt. %, preferably greater than 20 wt. % and particularly preferably greater than 30 wt. %. In accordance with one embodiment the fraction of refractory material is selected to be less than 80 wt. %, in accordance with a further embodiment less than 70 wt. % and in accordance with a further embodiment less than 60 wt. %.

In accordance with one embodiment the size according to the invention may comprise at least one floating agent. The floating agent increases the viscosity of the size in such a way that the solid constituents of the size do not sink in the suspension or only sink to a slight extent. Both organic and inorganic material, or else mixtures of these materials can be employed to increase viscosity. Examples of suitable inorganic floating agents include highly swellable clays.

In order to prevent sinking of the solid constituents and to simultaneously achieve uniform application over the casting mould, the viscosity is especially selected in a range from 1000 to 3000 mPas, particularly preferably 1200 to 2000 mPas.

The metal additive can then be distributed approximately homogeneously in the size and therefore also applied uniformly to a wall of a casting mould. The amount of metal additive applied to the surface of the casting mould can thus be controlled very precisely.

Both two-layer silicates and three-layer silicates can be used as highly swellable layer-lattice silicates, for example attapulgite, serpentines, kaolins, bentonites such as saponite, montmorillonite, beidellite and nontronite, vermiculite, illite, hectorite and mica. Hectorite also affords the size thixotropic properties, facilitating the formation of the protective layer on the casting mould since the size no longer flows after application. Since lattice-layer silicates contain water in the intermediate layers and this does not evaporate during application of the size to the hot casting mould having a temperature in the range from approximately 250 to 350° C., the amount of clay is preferably selected to be minimal. The amount of highly swellable lattice-layer silicate is preferably selected to be in the range from 0.01 to 5.0 wt. %, particularly preferably in the range from 0.1 to 1.0 wt. % based on the solid content of the size.

Organic thickening agents are preferably selected as the floating agent since they can be dried after application of the protective coating to such an extent that they hardly still release water upon contact with the liquid metal. Considered examples of organic floating agents include swellable polymers, such as caboxymethyl, methyl, ethyl, hydroxyethyl and hydroxypropyl celluloses, mucilages, polyvinyl alcohols, polyvinyl pyrrolidone, pectin, gelatin, agar agar and polypeptides, and alginates.

In accordance with a preferred embodiment the size according to the invention comprises at least one binder as a further constituent. The binder makes it possible to achieve better fixing of the size or, of the protective coating produced from the size to the wall of the casting mould. In addition, the mechanical stability of the protective coating is increased by the binder in such a way that low erosion under the influence of the liquid metal is observed. The binder preferably cures irreversibly in such a way that an abrasion-proof coating is obtained. Binders that do not resoften upon contact with ambient moisture are particularly preferred. All binders that are used in sizes may be contained. Both inorganic and organic binders can be used. For example clays, in particular bentonite can be used as a binder.

Curable binders are particularly used. For example, in the case of acrylate systems, curing can be achieved by radical formers that decompose when irradiated with high-energy radiation, for example ultraviolet radiation, with the formation of radicals.

Other exemplary binders include starches, dextrin, peptides, polyvinyl alcohol, polyvinyl acetate copolymers, polyacrylic acid, polystyrene and/or polyvinyl acetate polyacrylate dispersions. Binder systems are generally preferably used that can be introduced into aqueous, alcohol or aqueous-alcohol systems and do not resoften after curing under the effect of ambient moisture.

In accordance with a preferred embodiment an alkyd resin is used as a binder, preferably selected in such a way that it is soluble both in water and in low alcohols that preferably comprise 2 to 4 carbon atoms, such as ethanol, n-propanol and isopropanol.

In accordance with a further embodiment the coating mass according to the invention contains silica sol as a binder. The silica sol is preferably produced by neutralising water glass. The amorphous silica sol contained preferably has a specific surface in the range from 10 to 1000 m²/g, particularly preferably in the range from 30 to 300 m²/g.

The fraction of the binder is preferably in the range from 0.1 to 20 wt. %, particularly preferably 0.5 to 5 wt. % based on the solid weight of the size composition.

In accordance with a further preferred embodiment the size contains a graphite fraction. This assists the formation of lamellar carbon at the interface between the cast part and the casting mould. The graphite fraction is preferably in the range from 1 to 30 wt. %, particularly preferably 5 to 15 wt. % based on the weight of the size.

The size composition according to the invention can optionally also contain yet further components that are conventional for sizes, for example wetting agents, antifoamers, pigments, colorants or biocides. The fraction of these further constituents in the ready-to-use coating mass is preferably selected to be less than 1 wt. %.

For example anionic and non-anionic surfactants of medium and high polarity that have a HSB value of at least 7 can be used as wetting agents. An example of a wetting agent of this type is disodium dioctyl sulphosuccinate. The wetting agent is especially employed in an amount from 0.01 to 1 wt. %, preferably 0.05 to 0.3 wt. % based on the ready-to-use size composition.

Antifoamers or antifoaming agents can be used to prevent foam formation during production of the size composition or during application thereof. Foam formation during application of the size composition may lead to an uneven layer thickness and to holes in the coating. For example silicon or mineral oil can be used as antifoamers. The antifoamer is especially contained in an amount from 0.01 to 1 wt., preferably from 0.05 to 0.3 wt. % based on the ready-to-use size composition.

Pigments and colorants used conventionally may optionally be used in the size composition according to the invention. These are added in order to achieve a contrast, for example between different layers, or to produce a stronger separation effect of the size from the cast. Examples of pigments include red and yellow iron oxide as well as graphite. Examples of colorants include commercially available colorants, such as the Luconyl® colour range from BASF AG, Ludwigshafen, Germany. The colorants and pigments are especially contained in an amount from 0.01 to 10 wt. %, especially from 0.1 to 5 wt. % based on the solid content of the size composition.

In accordance with a further embodiment the size composition contains a biocide in order to prevent bacterial infection and therefore to avoid a negative effect on the rheology and binding force of the binder. This is particularly preferred if the carrier liquid contained in the size composition is formed substantially of water, i.e. the size composition according to the invention is provided in the form of a ‘water size’. Examples of suitable biocides include formaldehyde, 2-methyl-4-isothiazolin-3-one (MIT), 5-chloro-2-methyl-4-isothiazolin-3-one (CIT) and 1,2-benzisothiazolin-3-one (BIT). MIT, BIT or a mixture thereof are preferably employed. The biocides are normally used in an amount of 10 to 1000 ppm, especially from 50 to 500 ppm based on the weight of the ready-to-use size composition.

The solid content of the ready-to-use size composition is especially selected to be in the range from 10 to 60 wt. %, preferably 20 to 50 wt. %.

The size composition according to the invention can be produced in accordance with conventional methods. For example a size composition according to the invention can be produced by providing water and decomposing in it a clay acting as a floating agent with the use of a high-shear stirrer. The solid components, pigments and colorants as well as the metal additive are then stirred in until a homogeneous mixture is produced. Lastly, wetting agents, antifoaming agents, biocides and binders are stirred in.

The size composition according to the invention can be produced and distributed as a ready-to-use size. However, the size according to the invention can also be produced and distributed in concentrated from. In this instance the amount of carrier liquid necessary to provide the desired viscosity and density of the size is added to give a ready-to-use size. In addition, the size composition according to the invention can also be provided and distributed in the form of a kit, for example the solid components and the solvent components being present beside one another in separate containers. The solid components can be provided in a separate container as a pulverulent solid mixture. Further liquid components that may optionally be used, for example binders, wetting agents, wetters/antifoamers, pigments, colorants and biocides may also be present in this kit in a separate container. The solvent components may either comprise the components that are optionally to be used in addition, for example in a common container, or they can be provided separately from further optional constituents in a separate container. The suitable amounts of solid components, the optional further components and the solvent components are mixed together to produce a ready-to-use size. In the ready-to-use state a size according to the invention especially comprises a solid content of 20 to 80 wt. %, especially 30 to 70 wt. % based on the ready-to-use size composition. It is also possible to provide a size composition according to the invention in which the solvent components initially consist merely of water. A ready-to-use alcohol size can be provided from this water size, for example by adding a volatile alcohol or alcohol mixture, preferably ethanol, propanol, isopropanol and mixtures thereof, preferably in amounts from 40 to 200 wt. % based on the water size. The solid content of an alcohol size of this type is preferably 20 to 60 wt. %, preferably 30 to 40 wt. %.

Further characteristic parameters of the size composition can be adjusted depending on both the desired use of the size composition, for example as a base coating or as a top coating, and the desired layer thickness of the coating produced from the size composition. In a preferred embodiment size compositions according to the invention that are used to coat moulds and cores in foundry engineering thus have a viscosity from 11 to 25 s, more preferably 12 to 15 s (determined in accordance with DIN 53211; 4 mm flow cup, Ford Cup). Preferred densities of a ready-to-use size composition lie in the range from 0 to 120° Bé, more preferably from 30 to 50° Bé (determined in accordance with the Baumé buoyancy method; DIN 12791).

The size compositions according to the invention are adapted for coating casting moulds. The term ‘casting mould’ used here includes all types of bodies required to produce a cast part, such as cores, moulds and metal moulds. The use of size compositions according to the invention also includes a partial coating of casting moulds.

The invention also therefore relates to a method for producing a casting mould, in which at least one mould cavity provided in the casting mould is coated with the size composition according to the invention.

In the method:

-   -   a mould material mixture is provided that contains at least one         refractory mould material and a binder,     -   the mould material mixture is shaped to a basic mould comprising         a mould cavity; and     -   at least the faces of the mould cavity of the basic mould are         coated with a size composition as described above.

A basic mould is first produced in a known manner from a mould material mixture. In order to produce the mould mixture a refractory mould material is mixed with a binder and then shaped to a basic mould or part of a basic mould. The shape of the basic mould corresponds substantially to the casting mould or part of the casting mould. However, it does not comprise any coating with a size.

All refractory materials that are conventional for the production of moulds for the foundry industry can be used as a refractory mould material. Examples of suitable refractory mould materials include silica sand, zircon sand, olivine sand, aluminium silicate sand and chromite sand or mixtures thereof. Silica sand is preferably used. The refractory mould material should have a sufficient particle size so the mould produced from the mould material mixture exhibits sufficiently high porosity to make it possible for volatile compounds to escape during the casting process. At least 70 wt. %, particularly preferably at least 80 wt. % of the refractory mould material preferably has a particle size ≦290 μm. The mean particle size of the refractory mould material is preferably between 100 and 350 μm. For example the particle size can be ascertained by screen analysis. The refractory mould material is present in pourable form in such a way that a binder or liquid catalyst can be effectively applied to the particles of the refractory mould material, for example in a mixer.

In accordance with one embodiment regenerated used sands can be used as a refractory mould material. Larger aggregates are removed from the used sand and the grains of the used sand are optionally isolated. After mechanical or thermal treatment the used sands are dedusted and can then be used again. The acid balance of the regenerated used sand is preferably checked before it is used again. By-products contained in the sand, such as carbonates, may be converted into the corresponding oxides particularly during thermal regeneration and these oxides then react in an alkaline manner. If binders are used that are cured with catalysis by an acid, the acid added as a catalyst can, in this instance, be neutralised by the alkaline components of the regenerated used sand. For example in the case of mechanical regeneration of a used sand, acid may remain in the used sand and this must be taken into consideration when producing the binder since otherwise, for example, the processing time of the mould material mixture may be reduced.

The refractory mould material should be dry. The refractory mould material especially contains less than 1 wt. % water. The refractory mould material should not be too warm in order to prevent premature curing of the binder under the influence of heat. The refractory mould material should preferably be of a temperature in the range from 20 to 35° C. The refractory mould material can optionally be cooled or heated.

All binders that are conventional for the production of casting moulds for metal casting can be used as a binder. Both inorganic and organic binders can be used. For example water glass, which can be cured thermally or by the introduction of carbon dioxide, may be used as an inorganic binder. Exemplary organic binders include polyurethane no-bake and cold-box binders, binders based on furan resins or phenol resins, or else epoxy acrylate binders.

Binders based on polyurethanes are generally formed of two components, a first component containing a phenol resin and a second component containing a polyisocyanate. These two components are mixed with the refractory mould material and the mould material mixture is introduced into a mould by ramming, blowing, spraying or another method, compressed and then cured. Depending on the method used to introduce the catalyst into the mould material mixture, a distinction is made between the ‘polyurethane no-bake method’ and the ‘polyurethane cold-box method’.

In the no-bake method a liquid catalyst, generally a liquid tertiary amine is introduced into the mould material mixture before this mixture is introduced into a mould and cured. Phenol resin, polyisocyanate and a curing catalyst are mixed with the refractory mould material in order to produce the mould material mixture. For example it is possible to proceed in a manner in which the refractory mould material is initially enveloped by a component of the binder and for other components to then be added. The curing catalyst is added to one of the components. The finished, prepared mould material mixture must have a sufficiently long processing time so the mould material mixture can be plastically deformed for a sufficiently long period of time and processed to form a mould. Polymerisation must occur slowly in accordance with this so the mould material mixture does not cure in the storage containers or feed lines. On the other hand, curing must not occur too slowly so it is possible to achieve a sufficiently high throughput during the production of casting moulds. For example the processing time can be influenced by the addition of retarders, which slow down the curing process of the mould material mixture. For example a suitable retarder is phosphorous oxychloride.

In the cold-box method the mould material mixture is initially introduced into a mould without a catalyst. A gaseous tertiary amine is then guided through the mould material mixture and may optionally be mixed with an inert carrier gas. The binder binds very quickly upon contact with the gaseous catalyst in such a way that a high throughput is achieved during the production of casting moulds.

The binder systems based on polyurethanes contain a polyol component as well as a polyisocyanate component, in this instance it being possible to revert back to known components.

The polyisocyanate component of the binder may comprise an aliphatic, cycloaliphatic or aromatic isocyanate. The polyisocyanate especially contains at least 2 isocyanate groups, especially 2 to 5 isocyanate groups per molecule. Depending on the desired properties, mixtures of isocyanates can also be employed. The isocyanates used may consist of mixtures of monomers, oligomers and polymers and are therefore referred to hereinafter as polyisocyanates.

Any polyisocyanate that is conventional in polyurethane binders for mould material mixtures for the foundry industry can be employed as a polyisocyanate component. Suitable polyisocyanates include aliphatic polyisocyanates for example hexamethylene diisocyanate, alicyclic polyisocyanates such as 4,4′-dicyclohexylmethane diisocyanate, and dimethyl derivatives thereof. Examples of suitable aromatic polyisocyanates include toluene-2,4-diisocyanate, toluene-2,6-diisocyanate, 1,5-naphthaline diisocyanate, xylene diisocyanate and methyl derivatives thereof, diphenylmethane-4,4′-diisocyanate and polymethylene polyphenol polyisocyanate.

Although in principle all conventional polyisocyanates react with the phenol resin with formation of a cross-linked polymer structure, aromatic polyisocyanates are preferably employed, particularly preferably polymethylene polyphenol polyisocyanate, such as commercially available mixtures of diphenylmethane-4,4′-diisocyanate, its isomers and higher homologues.

The polyisocyanates can be employed both in a substance and dissolved in an inert or reactive solvent. A reactive solvent is understood to be a solvent that comprises a reactive group in such a way that it is integrated into the skeleton of the binder during setting of the binder. The polyisocyanates are preferably employed in diluted form in order to better envelope the particles of the refractory mould material with a thin film of the binder owing to the lower viscosity of the solution.

The polyisocyanates or their solutions in organic solvents are employed in sufficient concentrations to cure the polyol components, normally in a range from 10 to 500 wt. % based on the weight of the polyol components. 20 to 300 wt. % based on the same principle are preferably employed. Liquid polyisocyanates can be employed in undiluted form, whilst solid or viscous polyisocyanates are dissolved in organic solvents. Up to 80 wt. %, especially up to 60 wt. % and particularly preferably up to 40 wt. % of the isocyanate component may consist of solvents.

The polyisocyanate is preferably employed in such an amount that the number of isocyanate groups is 80 to 120% based on the number of free hydroxyl groups of the polyol component.

All polyols used in polyurethane binders can be employed as a polyol component. The polyol component contains at least 2 hydroxyl groups that can react with the isocyanate groups of the polyisocyanate component in order to cross-link the binder during curing and thus achieve greater strength of the cured mould.

Phenol resins that are obtained by condensation of phenols with aldehydes, preferably formaldehyde in liquid phase at temperatures up to 180° C. in the presence of a catalytic amount of metal are preferably used as polyols. The methods for producing phenol resins of this type are known per se.

The polyol component is preferably employed in a liquid state or dissolved in organic solvents so as to make it possible to achieve a homogeneous distribution of the binder over the refractory mould material. The polyol component is preferably employed in anhydrous form since the reaction of the isocyanate component with water is an undesired side reaction. In this context non-aqueous or anhydrous means a water content of the polyol component of preferably less than 5 wt. %, preferably less than 2 wt. %.

‘Phenol resin’ is understood to be the reaction product of phenol, phenol derivatives, bisphenols and higher phenol condensation products with an aldehyde. The composition of the phenol resin is dependent on the specifically selected starting materials, the ratio of the starting material and the reaction conditions. For example the type of catalyst, reaction time and reaction temperature thus play an important role as well as the presence of solvents and other substances.

The phenol resin is typically present as a mixture of different compounds and may contain addition products, condensation products and unreacted starting compounds, such as phenols, bisphenol and/or aldehyde in very different ratios.

An ‘addition product’ is understood to mean reaction products in which an organic component substitutes at least one hydrogen on a previously unsubstituted phenol or a condensation product. ‘Condensation product’ is understood to mean reaction products with two or more phenol rings.

Phenol resins are produced as condensation reactions of phenols with aldehydes and, depending on the quantitative proportions of educts, the reaction conditions and catalysts used, can be divided into two product classes: novolacs and resols:

Novolacs are soluble, meltable, non-self-curing and storage-stable oligomers with a molecular weight in the range of approximately 500 to 5,000 g/mol. They are formed during the condensation of aldehydes and phenols in a molar ration of 1:>1 in the presence of acidic catalysts. Novolacs are methylol-group-free phenol resins in which the phenyl cores are linked via methylene bridges. They can be cured at increased temperatures with cross-linking after the addition of curing agents, such as formaldehyde-releasing agents, preferably hexamethylenetetramine.

Resols are mixtures of hydroxymethyl phenols that are linked via methylene and methylene ether bridges and are obtainable by reacting aldehydes and phenols in a molar ratio of 1:1<1, optionally in the presence of a catalyst, for example an alkaline catalyst. They have a molecular weight M_(w) of ≦10,000 g/mol.

The phenol resins that are particularly suitable as the polyol component are known under the name “o-o′” or “high-ortho” novolacs or benzyl ether resins. They are obtainable by condensation of phenols with aldehydes in a weak acidic medium with the use of suitable catalysts.

Suitable catalysts for producing benzyl ether resins include salts of divalent ions of metals, such as Mn, Zn, Cd, Mg, Co, Ni, Fe, Pb, Ca and Ba. Zinc acetate is preferably used. The amount used is not critical. Typical amounts of metal catalyst are 0.02 to 0.3 wt. %, preferably 0.02 to 0.15 wt. % based on the total amount of phenol and aldehyde.

All phenols used conventionally are suitable for the production of phenol resins. Substituted phenols or mixtures thereof are employed in addition to unsubstituted phenols. The phenol compounds are either not substituted in both ortho positions or are not substituted in one ortho position and in the para position in order to enable polymerisation. The remaining ring carbon atoms can be substituted. The selection of the substituents is not particularly limited provided the substituent does not affect polymerisation of the phenol or aldehyde in a detrimental manner. Examples of substituted phenols include alkyl-substituted phenols, alkoxy-substituted phenols and aryloxy-substituted phenols.

For example the above-mentioned substituents have 1 to 26, preferably 1 to 15 carbon atoms. Examples of suitable phenols include o-cresol, m-cresol, p-cresol, 3,5-xylene, 3,4-xylene, 3,4,5-trimethylphenol, 3-ethylphenol, 3,5-diethylphenol, p-butylphenol, 3,5-dibutylphenol, p-amylphenol, cyclohexylphenol, p-octylphenol, p-nonylphenol, 3,5-dicyclohexylphenol, p-crotylphenol, p-phenylphenol, 3,5-dimethoxyphenol and p-phenoxyphenol.

Phenol itself is particularly preferred. Higher condensed phenols, such as bisphenol A are also suitable. In addition, multivalent phenols that have more than one phenol hydroxyl group are also suitable. Preferred multivalent phenols include 2 to 4 phenol hydroxyl groups. Special examples of suitable multivalent phenols include catechol, resorcin, hydroquinone, pyrogallol, fluoroglycine, 2,5-dimethyl resorcin, 4,5-dimethyl resorcin, 5-methyl resorcin or 5-ethyl resorcin.

Mixtures of different monovalent and multivalent and/or substituted and/or condensed phenol components can also be used for the production of the polyol component.

In one embodiment phenols of the general formula I:

are used to produce the phenol resin component, in which A, B and C, independently of one another, are selected from a hydrogen atom, a branched or linear alkyl radical that, for example, may contain 1 to 26, preferably 1 to 15 carbon atoms, a branched or linear alkoxy radical that, for example, may contain 1 to 26, preferably 1 to 15 carbon atoms, a branched or linear alkenoxy radical that, for example, may contain 1 to 26, preferably 1 to 15 carbon atoms, or an aryl or alkylaryl radical, for example bisphenyls.

Aldehydes of the formula:

R—CHO,

in which R is a hydrogen atom or a carbon atom radical with preferably 1 to 8, particularly preferably 1 to 3 carbon atoms, are suitable for the production of the phenol resin component. Special examples include formaldehyde, acetaldehyde, propionaldehyde, furfurylaldehyde and benzaldehyde. Formaldehyde is particularly preferably used, either in its aqueous form, as para-formaldehyde or trioxane.

An at least equivalent molar number of aldehyde based on the molar number of the phenol component should be used in order to obtain the phenol resins. The molar ratio of aldehyde to phenol is preferably 1:1.0 to 2.5:1, particularly preferably 1.1:1 to 2.2:1, particularly preferably 1.2:1 to 2.0: 1.

The phenol component is produced by methods known to the person skilled in the art. The phenol and the aldehyde are converted in substantially anhydrous conditions in the presence of a divalent metal ion at temperatures of preferably less than 130° C. The water produced is distilled off. A suitable entrainer, for example toluene or xylene, can be added to the reaction mixture or distillation is carried out at reduced pressure.

The phenol component is reacted with an aldehyde, preferably to form benzyl ether resins for the binder of the mould material mixture. Reaction with a primary or secondary aliphatic alcohol to form an alkoxy-modified phenol resin is also possible in single-step or two-step methods (EP-B-0 177 871 and EP 1 137 500). In single-step methods the phenol, aldehyde and alcohol are reacted in the presence of a suitable catalyst. In two-step methods an unmodified resin is first produced that is then reacted with an alcohol. When using alkoxy-modified phenol resins there is no restriction on the molar ratio, but the alcohol component is preferably used in a molar ration of alcohol:phenol of less than 0.25 in such a way that less than 25% of the hydroxymethyl groups are etherified. Suitable alcohols include primary and secondary aliphatic alcohols with a hydroxyl group and 1 to 10 carbon atoms. Examples of suitable primary and secondary alcohols include methanol, ethanol, propanol, n-butanol and n-hexanol. Methanol and n-butanol are particularly preferred.

The phenol resin is preferably selected in such a way that cross-linking with the polyisocyanate component is possible. Phenol resins that comprise molecules with at least two hydroxyl groups are particularly suitable for construction of a network. The phenol resin component or isocyanate component of the binder system is preferably employed as a solution in an organic solvent or a combination of organic solvents. Solvents may be necessary in order to keep the components of the binder in a sufficiently poorly viscous state. This is necessary, inter alia, in order to obtain uniform cross-linking of the refractory mould material and pourability thereof.

All solvents that are conventionally used in binder systems of this type for foundry engineering can be employed as solvents for the polyisocyanate or polyol components of the binder system based on polyurethanes. For example oxygen-rich, polar organic solvents are suitable as solvents. Above all, dicarboxylic acid ester, glycol ether ester, glycol diester, glycol diether, cyclic ketones, cyclic esters or cyclic carbonates are suitable. Dicarboxylic acid esters, cyclic ketones and cyclic carbonates are preferably used. Dicarboxylic acid esters have the formula R^(a)OOC—R^(b)—COOR^(a) in which the radicals R^(a) each represent, independently of one another, an alkyl group with 1 to 12, preferably 1 to 6 carbon atoms, and R^(b) is an alkylene group, i.e. a divalent alkyl group with 1 to 12, preferably 1 to 6 carbon atoms. R^(b) may also comprise one or more carbon-carbon double bonds. Examples include dimethyl esters of carboxylic acids with 4 to 10 carbon atoms that are obtainable, for example, under the name “dibasic ester” (DBE) from Invista International S.à.r.1., Genf, Switzerland. Glycol ether esters are compounds of the formula R^(c)—O—R^(d)—OOCR^(e), in which R^(c) is an alkyl group with 1 to 4 carbon atoms, R^(d) is an ethylene group, a propylene group or an oligomer ethylene oxide or propylene oxide and R^(e) is an alkyl group with 1 to 3 carbon atoms. Gycol ether acetates, for example butylgycol acetate are preferred. Gycol diesters correspondingly have the general formula R^(e)COO—R^(d)OOCR^(e), in which R^(d) and R^(e) are as defined above and the radicals R^(e) are each selected independently of one another. Gycol diacetates, for example propylene glycol diacetate are preferred. Gycol diethers can be characterised by the formula R^(c)—O—R^(d)—O—R^(c), in which R^(c) and R^(d) are as defined above and the radicals R^(c) are each selected independently of one another. A suitable glycol diether is, for example, dipropylene glycol dimethylether. Cyclic ketones, cyclic esters and cyclic carbonates with 4 to 5 carbon atoms are also suitable. For example a suitable cyclic carbonate is propylene carbonate. The alkyl and alkylene groups may each be branched or linear.

The fraction of the solvent in the binder system is preferably selected to be not too high since the solvent evaporates during the production and application of the mould produced from the mould material mixture, and can therefore lead for example to an annoying bad odour or to smoke formation during casting. The fraction of the solvent in the binder system is preferably selected to be less than 50 wt. %, particularly preferably less than 40 wt. %, in particular preferably less than 35 wt. %.

As described above, the binder is first mixed with the refractory mould material to form a mould material mixture in order to produce the mould. If the mould is produced by the PU no-bake method, a suitable catalyst may already be added to the mould material mixture. Liquid amines are preferably added to the mould material mixture. These amines preferably have a pK_(b) value of 4 to 11. Examples of suitable catalysts include 4-alkyl pyridine, in which the alkyl group comprises 1 to 4 carbon atoms, isoquinoline, acrylpyridine such as phenylpyridine, pyridine, acryline, 2-methoxypyridine, pyridazine, 3-chloropyridine, quinoline, n-methylimidazole, 4,4′-dipyridine, phenylpropyl pyridine, 1-methylbenzimidazole, 1,4-thiazine, N,N-dimethylbenzylamine, triethylamine, tribenzylamine, N,N-dimethyl-1,3-propanediamine, N,N-dimethylethanolamine and triethanolamine. The catalyst may optionally be diluted with an inert solvent, for example 2,2,4-trimethyl-1,3-pentadiol-diisobutyrate, or a fatty acid ester. The amount of catalyst added is selected to be in the range of 0.1 to 15 wt. % based on the weight of the polyol component.

The mould material mixture is then introduced into a mould using conventional means and compressed there. The mould material mixture is then cured to form a mould. The mould should preferably maintain its outer shape during curing.

If the curing process takes place by the PU cold-box method, a gaseous catalyst is guided through the shaped mould material mixture. Conventional catalysts in the field of the cold-box method can be used as a catalyst. Amines are particularly preferably used as catalysts, in particular preferably dimethylethylamine, dimethyl-n-propylamine, dimethylisopropylamine, dimethyl-n-butylamine, triethylamine and trimethylamine in their gaseous form or as an aerosol.

In accordance with a further preferred embodiment a furan resin or a phenol resin is employed as a binder, the mould material mixture being cured in accordance with the “furan no-bake” method with catalysis by a strong acid.

Furan and phenol resins exhibit very good decomposition properties during casting. The furan or phenol resin decomposes under the influence of the heat of the liquid metal and the strength of the casting mould is lost. After the casting process, cores can therefore be very easily emptied from cavities, optionally after prior vibration of the cast part.

The reactive furan resins contained as a first component in “furan no-bake binders” comprise furfuryl alcohol as a main component. Furfuryl alcohol can react with itself under acidic catalysis and form a polymer. Pure furfuryl alcohol is not generally used for the production of furan no-bake binders, but instead further compounds that are polymerised in the resin are added to the furfuryl alcohol. Examples of compounds of this type include aldehydes, such as formaldehyde or furfural, ketones such as acetone, phenols, urea or else polyols such as sugar alcohols or ethylene glycol. Yet further components that influence the properties of the resin, for example the elasticity thereof, can be added to the resins. For example melamine can be added to bind free formaldehyde.

Furan no-bake binders are usually formed by first producing furfuryl-containing precondensates from, for example, urea, formaldehyde and furfuryl alcohol in acidic conditions. The reaction conditions are selected in such a way that only slight polymerisation of the furfuryl alcohol occurs. These precondensates are then diluted with furfuryl alcohol. Resols can also be used to produce furan no-bake binders. Resols are produced by polymerisation of mixtures of phenol and formaldehyde. These resols are then diluted with furfuryl alcohol.

The second component of the furan no-bake binder forms an acid. On the one hand this acid neutralises alkaline components that are contained in the refractory mould material and, on the other hand, catalyses the cross-linking of the reactive furan resin.

Aromatic sulphonic acids and, in some special cases, also phosphoric acid or sulphuric acid are usually used as acids. Phosphoric acid is used in concentrated form, i.e. in concentrations of more than 75%. However, it is only adapted for the catalytic curing of furan resins with a relatively high fraction of urea. The nitrogen content of resins of this type is more than 2.0 wt. %. Sulphuric acid can be added to the furan resins of weaker acids as a relatively strong acid as a starter for the curing process. However, a smell that is typical of sulphur compounds develops during casting. In addition there is the risk that sulphur will be absorbed by the casting material and will influence the properties thereof. Aromatic sulphonic acids are usually employed as catalysts. Above all toluene sulphonic acid, xylene sulphonic acid and benzene sulphonic acid are used owing to their good availability and their high acid strength.

As the second largest group of acid-catalysed curable no-bake binders, phenol resins contain resols, i.e. phenol resins produced with an excess of formaldehyde, as a reactive resin component. Phenol resins exhibit considerably lower reactivity compared to furan resins and require strong sulphonic acids as catalysts. Phenol resins exhibit relatively high viscosity that increases during longer periods of storage of the resin. The viscosity increases rapidly particularly at temperatures below 20° C. in such a way that the sand must be heated in order to be able to apply the binder uniformly to the surface of the sand grains. Once the phenol no-bake binder has been applied to the refractory mould material, the mould material mixture should be processed as thoroughly as possible so as to not have to deal with deterioration of the quality of the mould material mixture caused by premature curing, which may lead to a deterioration in the strength of the casting moulds produced from the mould material mixture. When using phenol no-bake binders, the flowability of the mould material mixture is generally poor. When producing the casting mould, the mould material mixture must therefore be compressed carefully in order to achieve a high level of strength of the casting mould.

The mould material mixture should be produced and processed at temperatures in the range from 15 to 35° C. At excessively low temperatures the mould material mixture cannot be processed easily owing to the high viscosity of the phenol no-bake resin. At temperatures above 35° C. the processing time is shortened by premature curing of the binder.

After casting, mould material mixtures based on phenol no-bake binders can also be reprocessed, mechanical or thermal or combined mechanical/thermal methods possibly being used in this instance also.

An acid is applied to the pourable refractory material, an acid-coated refractory mould material being obtained. The acid is applied to the refractory mould material using conventional methods, for example by spraying the acid onto the refractory mould material. The amount of acid is preferably selected to be in the range from 5 to 45 wt. %, particularly preferably in the range from 20 to 30 wt. % based on the weight of the binder and determined as pure acid, i.e. with no consideration of a solvent that may have been used. If the acid is not already present in liquid form and has a sufficiently low viscosity to be distributed over the particles of the refractory mould material in the form of a thin film, the acid is dissolved in a suitable solvent. Exemplary solvents include water or alcohols or mixtures of water and alcohol. However, particularly when using water, the solution is produced as concentrated as possible so as to keep the amount of water introduced into the binder or mould material mixture to a minimum. The mixture is thoroughly homogenised from the refractory mould material and acid to provide uniform distribution of the acid over the particles.

A binder that is curable by acid is then applied to the refractory mould material coated with acid. The amount of binder is preferably selected to be in the-range from 0.25 to 5 wt. %, particularly preferably in the range from 1 to wt. % based on the refractory mould material and determined as resin component. All binders that are curable by acid, in particular those binders curable by acid that are already conventional for the production of mould material mixtures for the foundry industry can be used as a binder curable by acid. In addition to a cross-linkable resin, the binder may also contain further conventional components, for example solvents for adjusting the viscosity or extenders that replace part of the cross-linkable resin.

The binder is applied to the refractory mould material coated with acid and by moving the mixture is distributed over the particles of the refractory mould material in the form of a thin film.

The amounts of binder and acid are selected in such a way that, on the one hand, a sufficient strength of the casting mould is achieved and, on the other hand, a sufficient processing time of the mould material mixture is achieved. For example a processing time in the range from 5 to 45 minutes is suitable.

The refractory mould material coated with the binder is then shaped by conventional methods to form a mould. For this purpose the mould material mixture can be introduced into a suitable mould and compressed there. The mould obtained is then left to cure.

All furan resins as already used in furan no-bake binder systems can be used as furan no-bake binders.

The furan resins employed in technical furan no-bake binders are usually precondensates or mixtures of furan resins with further monomers or precondensates. The precondensates contained in furan no-bake binders are produced in a manner known per se.

In accordance with a preferred embodiment furfuryl alcohol is employed in combination with urea and/or formaldehyde or urea/formaldehyde precondensates. Formaldehyde can be employed both in monomeric form, for example in the form of a formalin solution, and in the form of its polymers, such as trioxane or paraformaldehyde. Other aldehydes or else ketones can also be used in addition to or instead of formaldehyde. Examples of suitable aldehydes include acetaldehyde, propionaldehyde, butyraldehyde, acrolein, crotonaldehyde, benzaldehyde, salicylaldehyde, cinnamaldehyde, glyoxal and mixtures of these aldehydes. Formaldehyde is preferred and is preferably employed in the form of paraformaldehyde.

All ketones that exhibit sufficiently high reactivity can be used as ketone components. Exemplary ketones include methyl ethyl ketone, methyl propyl ketone and acetone, acetone being preferably used.

The aldehydes and ketones named can be employed as individual compounds or else mixed together.

The molar ratio of aldehyde, particularly formaldehyde, or ketone to furfuryl alcohol may be selected within wide ranges. When producing the furan resins especially 0.4 to 4 mol of furfuryl alcohol, preferably 0.5 to 2 mol furfuryl alcohol can be used per mol of aldehyde.

Furfuryl alcohol, formaldehyde and urea can be heated to boiling point to produce the precondensates, for example after adjustment of a pH value of more than 4.5, water being continuously distilled off from the reaction mixture. The reaction time can be a number of hours, for example 2 hours. In these reaction conditions there is practically no polymerisation of the furfuryl alcohol. However, the furfuryl alcohol is condensed in a resin together with the formaldehyde and the urea.

In accordance with an alternative method furfuryl alcohol, formaldehyde and urea are reacted under heat at a pH value of considerably less than 4.5, for example at a pH of 2.0, the water produced during the condensation process possibly being distilled off at reduced pressure. The reaction product exhibits relatively high viscosity and is diluted with furfuryl alcohol for production of the binder until the desired viscosity is reached.

Combinations of these production methods can also be implemented.

It is also possible to introduce phenol into the precondensate. For this purpose the phenol may first be reacted with formaldehyde in alkaline conditions to form a resol resin. This resol can then be reacted or mixed with furfuryl alcohol or a furan-group-containing resin. For example furan-group-containing resins of this type can be obtained by the above-described methods. Higher phenols can also be used for the production of the precondensate, for example resorcin, cresol or else bisphenol A. The fraction of the phenol or higher phenols in the binder is especially selected to be in the range of up to 45 wt. %, preferably up to 20 wt. % and particularly preferably up to 10 wt. %. In accordance with one embodiment the fraction of the phenol or higher phenols can be selected to be greater than 2 wt. %, in accordance with a further embodiment greater than 4 wt. %.

Furthermore it is also possible to use condensates of aldehydes and ketones that are mixed with furfuryl alcohol for production of the binder. Condensates of this type can be produced by reacting aldehydes and ketones in alkaline conditions. Formaldehyde, particularly in the form of paraformaldehyde is preferably used as aldehyde. Acetone is preferably employed as ketone. Other aldehydes or ketones can also be used however. The relative molar ratio of aldehyde to ketone is preferably selected to be in the range from 7:1 to 1:1, preferably 1.2:1 to 3.0:1. Condensation is preferably carried out in alkaline conditions at pH values in the range from 8 to 11.5, preferably 9 to 11. For example a suitable base is sodium carbonate.

The amount of furfuryl alcohol contained in the furan no-bake binder is, on the one hand, determined by the effort to keep the fraction to a minimum for cost reasons. On the other hand, the strength of the casting mould is improved by a high fraction of furfuryl alcohol. However, very brittle casting moulds that are difficult to work with are produced with a very high fraction of furfuryl alcohol in the binder. The fraction of furfuryl alcohol in the binder is especially selected to be in the range from 30 to 95 wt. %, preferably 50 to 90 wt. % and particularly preferably 60 to 85 wt. %. The fraction of urea and/or formaldehyde in the binder is especially selected to be in the range from 2 to 70 wt. %, preferably 5 to 45 wt. % and particularly preferably 15 to 30 wt. %. The fraction includes both the unbound fractions of these compounds contained in the binder and the fractions that are bound in the resin.

Further additives can be added to the furan resins, such as ethylene glycol or similar aliphatic polyols, for example sugar alcohols such as sorbitol that act as extenders and replace some of the furfuryl alcohol. If too much of these extenders are added this may lead, in the worst case scenario, to a reduction in the strength of the casting mould and to a reduction in reactivity. The fraction of this extender in the binder is thus especially selected to be less than 25 wt. %, preferably less than 15 wt. % and particularly preferably less than 10 wt. %. In order to save costs without having to deal with a considerable influence on the strength of the casting mould, the fraction of the extender is selected to be greater than 5 wt. % in accordance with one embodiment.

The furan no-bake binder can also contain water. However since water slows down the curing process of the mould material mixture and is produced as a reaction product during curing, the fraction of water is preferably selected to be minimal. The fraction of water in the binder is especially less than 20 wt. %, preferably less than 15 wt. %. From an economical point of view, an amount of water of more than 5 wt. % can be tolerated in the binder.

In the method according to the invention resols are used as phenol resins. Resols are mixtures of hydroxymethyl phenols that are linked via methylene and methylene ether bridges and are obtainable by reacting aldehydes and phenols in a molar ratio of 1:<1, optionally in the presence of a catalyst, for example an alkaline catalyst. They have a molecular weight M_(w) of ≦10,000 g/mol.

All phenols used conventionally are suitable for the production of the phenol resins, phenol being particularly preferred. Formaldehyde is preferably employed as an aldehyde component, particularly in the form of paraformaldehyde. Alternative phenols and aldehydes have already been described in conjunction with the polyurethane binders. Reference is made to the relevant passages.

The binders may contain further conventional additives, for example silanes as coupling agents. Examples of suitable silanes include aminosilanes, epoxysilanes, mercaptosilanes, hydroxysilanes and ureido silanes such as γ-hydroxy propyl trimethoxysilane, γ-aminopropyltrimethoxysilane, 3-ureido propyl triethoxysilane, γ-mercaptopropyltrimethoxysilane, γ-glycidoxypropyltrimethoxysilane, β-(3,4-epoxycyclohexyl)trimethoxysilane, and N-=62 -(aminoethyl)-γ-aminopropyltrimethoxysilane.

If a silane of this type is used, it is added to the binder in an amount of 0.1 to 3 wt. %, preferably 0.1 to 1 wt. %.

The binders can also contain yet further conventional components, for example activators or plasticisers.

The mould material mixture can also contain yet further conventional constituents in addition to the refractory mould material, the binder and optionally the catalyst. Exemplary further constituents include iron oxide, ground flax fibres, wood dust granules, ground coal or clay.

The mould material mixture is then shaped to form a basic mould or part of a basic mould using conventional methods and is optionally cured. The basic mould can then be assembled completely or in part and the mould cavity provided in the basic mould can be coated with the above-described size composition, either completely or over portions. Conventional methods can be used for this. For example the size composition can be applied by a dipping method, flow coating, painting on or by spraying.

If the dipping method is used as the application method then the casting mould of which the mould cavity has optionally been covered with a basic coating is dipped in a container filled with a ready-to-use size composition according to the invention for approximately 2 seconds to 2 minutes. The casting mould is then removed from the size composition and excess size composition is drained off from the casting mould. The time taken for the excess size composition to drain off after dipping depends on the runoff behaviour of the size composition used.

If the spraying method is used as the application method then commercially available pressure vessel spraying devices are used. In this instance a pressure vessel is filled with the size composition in the diluted state. The size can be pressed into a spraying gun via the overpressure to be set and is then sprayed with the aid of an atomiser that can be adjusted separately. The conditions during spraying are preferably selected in such a way that the pressure for the size composition and atomiser is adjusted at the gun in such a way that the sprayed size composition is still wet when it contacts the mould or core, but is applied uniformly.

The carrier liquid contained in the size is then evaporated in such a way that a dry size layer is obtained. All conventional drying methods can be used as a drying method, for example open-air drying, drying with dehumidified air, drying with microwave or infrared radiation, drying in convection ovens and comparable methods.

In a preferred embodiment of the invention the coated casting mould is dried at 100 to 250° C., preferably at 120 to 180° C. in a convection oven. When using alcohol sizes, the size composition according to the invention is preferably dried by burning off the alcohol or alcohol mixture. In this instance the coated casting mould is additionally heated by the combustion heat. In a further preferred embodiment the coated casting mould is dried in the open air with no further treatment.

The size layer can then optionally be cured, for example by irradiation with UV radiation, if an appropriate curable binder is contained in the size composition.

The size may be applied in the form of an individual layer or else in the form of a plurality of superposed layers. The individual layers may be of identical or different composition. For example a base coating may first be produced from a commercially available size that contains no metal additive according to the invention. For example water sizes or else alcohol sizes may be used as a base coating. However, it is also possible for all layers to be produced from the size composition according to the invention. The layer that will later come into contact with the liquid metal is, however, always produced from the size according to the invention. If a plurality of layers are applied, each individual layer can be dried either completely or in part after application.

The coating produced from the size composition especially comprises a dry layer thickness of at least 0.1 mm, preferably at least 0.2 mm, particularly preferably at least 0.55 mm. In accordance with one embodiment the thickness of the coating is selected to be less than 1.5 mm. In this instance the dry layer thickness is the layer thickness of the dried coating obtained by drying the size composition by substantially completely removing the solvent components and optionally subsequent curing. The dry layer thickness of the base coating and of the top coating are especially ascertained by measurement with the wet-film thickness comb.

The casting mould can then optionally be assembled completely.

The invention further relates to the use of the above-described casting mould to produce a cast part.

For this purpose a casting mould is first provided. This may be a dead mould produced from a refractory material, for example silica sand, and a binder, as described above, or else a permanent mould as is used conventionally to 1 produce pipes, bearings or sleeves, the mould cavity being lined with the above-described size composition. The casting mould comprises a protective coating, at least on those faces that come into contact with the liquid metal, which coating insulates the liquid metal from the casting mould and can positively influence the nature of the surface of the cast part. Liquid metal, especially iron or an iron alloy is now introduced into the prepared casting mould. The liquid metal is then left to set to form a cast part and then the cast part is separated from the casting mould. Conventional methods are implemented for this purpose. With dead moulds the casting mould is mechanically destroyed, for example by vibration. With permanent moulds the cast part is removed from the casting mould using conventional methods.

The invention further relates to a casting mould that has a mould coating produced from the above-described size. A casting mould of this type advantageously comprises insulation between the liquid metal and the casting mould, whereby the thermal load of the casting mould during the casting process is reduced. As a further advantage the mould coating comprises a metal additive that can positively influence the surface properties of the cast part and, in particular, represses the formation of a maculate surface on the cast part.

Casting moulds that comprise a top coating produced from the size composition according to the invention are used, inter alia, to produce wind turbine hubs, grinding bowls, engines and engine components, machine beds and turbines, general machine components or pressing tools.

The invention will now be explained in greater detail with reference to examples.

EXAMPLE 1

The core size used in the examples below had the composition given in Table 1.

TABLE 1 Composition of the size Component Wt. % zircon silicate 75 μm 50.00 manganese (325 mesh) 20.00 clay mineral 03.00 synthetic resins 02.00 rheological additives 00.50 ethanol 14.50 isopropanol 10.00

The mould casting size was produced as follows: isopropanol is provided and the clay is decomposed therein by use of a high-shear stirrer for at least 15 minutes. The refractory components, Pigments, manganese and colorants are then stirred in for at least 15 minutes until a homogeneous mixture is produced. Lastly, ethanol, rheological additives and binders are stirred in. 

1. Coating composition for casting moulds and cores comprising at least one carrier liquid and a metal additive that comprises a metal or a compound of a metal, wherein the metal is selected from the group of manganese and copper, and wherein the fraction of the metal additive is less than 50 wt. % based on the solid content of the coating composition.
 2. Coating composition according to claim 1, wherein the metal is manganese or the compound of a metal comprising manganese.
 3. Coating composition according to either claim 1, wherein the metal is employed in pure form or in the form of an alloy with other metals.
 4. Coating composition according to claim 1, wherein the metal or metal compound is contained in the metal additive in a fraction of at least 10 wt. %, determined as metal and based on the weight of the metal additive.
 5. Coating composition according to claim 1, wherein the metal additive is contained in the coating composition in a fraction of at least 10 wt. % based on the solid fraction of the coating composition.
 6. Coating composition according to claim 1, wherein the metal is contained in the metal additive in the form of an iron alloy.
 7. Coating composition according to claim 1, wherein the metal additive has a mean particle size (D50) in the range from 0.5 to 5000 μm.
 8. Coating composition according to claim 1, wherein the coating composition contains a solvent and the solvent is formed, at least in part, of at least one alcohol.
 9. Coating composition according to claim 8, wherein the at least one alcohol forms a fraction of at least 50 wt. % in the solvent.
 10. A method for producing a casting mould, wherein a mould material mixture is provided that contains at least one refractory mould material and a binder, the mould material mixture is shaped to a basic mould comprising a mould cavity, and at least the faces of the mould cavity of the basic mould are coated with a coating composition according to claim
 1. 11. A casting mould with a mould cavity, wherein at least the faces of the mould cavity are coated with a coating according to claim
 1. 12. Use of a casting mould according to claim 11 for metal casting.
 13. The use according to claim 12, wherein the metal casting is an iron casting or a steel casting.
 14. The coating composition according to claim 1, wherein the fraction of the metal additive in the coating composition is less than 40 wt. %, based on the solid content of the coating composition.
 15. The coating composition according to claim 1, wherein the composition further contains a binder.
 16. The coating composition according to claim 1, wherein the fraction of the metal additive in the coating composition is less than 35 wt. % based on the solid content of the coating composition. 